<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    // 问题
    alert(1420038345634918421);//1420038345634918400
    // 解决办法
    alert("1420038345634918421");//1420038345634918421
</script>
</body>
</html>